1. Field of the Invention
The present invention relates to analysis of the isotopic concentration of an element. In particular, the present invention relates to analysis of the concentration of hydrogen-isotope-containing mixtures. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Isotopes are atoms of the same element that differ in their number of neutrons; thus, they have slightly different masses. It is often desirable to measure the concentrations of isotopes in a sample containing one or more elements in a mixture, that is, the relative amounts of the isotopes of each element which are found in the sample. The isotopic concentration of hydrogen in particular is important. Hydrogen has three isotopes, called protium (H), deuterium (D), and tritium (T) to reflect the number of neutrons in their respective nuclei (one, two and three, respectively). Molecular hydrogen may include the combinations HD, HT, and DT as well as H.sub.2, D.sub.2, and T.sub.2. The isotopic concentration of a hydrogen sample thus may include any one or some combination of these possibilities. (The term "hydrogen" is used throughout this specification in the elemental sense rather than the isotopic sense.)
Deuterium and tritium, in particular, are useful and valuable isotopes; they are also radioactive isotopes. In nuclear material production processes, it is important to determine the relative amounts of the various isotopes that are produced, as well as to monitor the facilities to assure worker safety. The concentrations of hydrogen isotopes in air are frequently measured in the course of environmental testing and monitoring. The ratio of deuterium to hydrogen (D/H ratio) is useful in dating soil and minerals exposed to solar wind irradiation, in analysis of planetary atmospheric gases, and, in analyzing the solar wind, to provide a limit for the D/H ratio in the sun (see Low, U.S. Pat. No. 3,666,942).
Methods for determination of hydrogen isotope concentration include deuterium separation by gas chromatography, mass spectrometry, high-resolution techniques such as Fourier Transform Mass Spectrometry (FTMS), and infrared absorption analysis of gas samples.
Low et al. (U.S. Pat. No. 3,666,942) describe a method for determining the amount of deuterium in a mixture containing hydrogen and low concentrations of deuterium, such as might be found in typical terrestrial hydrogen samples. A sample of the mixture is passed through a hydrogen-selective film to remove gases other than hydrogen. The hydrogen is carried to a chromatographic detector for a measurement of total hydrogen content, then to an ionization chamber where it is ionized to form an effluent containing HD.sup.+ ions. The effluent is delivered to a high resolution infrared detector whose output signal depends on the HD.sup.+ concentration. The ratio of the two signals is proportional to the H/D ratio of the initial sample.
A quadrupole mass spectrometer (QMS) can be used for separating isotopes, as in the system described by Habfast et al. (U.S. Pat. No. 5,043,575). A quadrupole mass filter is provided with a voltage supply device controlled by the ion separation system. The supply voltages of the filter are switched over synchronously with the varying mass deflection setting of the ion separation system, thereby controlling the range of masses allowed through the filter.
Pokar et al. (U.S. Pat. No. 4,039,828) show a QMS system for analyzing corrosive gases or gases tending to sublimate. The sample beam is collimated by an aperture before it enters the ionization chamber. The gas molecules which do not pass through the aperture are condensed out onto a liquid nitrogen cryopump, or "cooling finger."
Many of the presently-available methods for analyzing hydrogen isotope concentrations suffer from poor resolution due to interactions between the ions in the ion source or ion separator, homogeneity errors, and other sources of error. High-resolution systems such as FTMS are often prohibitively expensive: a typical FTMS system can cost ten times as much as a quadrupole mass spectrometer (QMS). Furthermore, adsorption of hydrogen isotopes on the walls of the system may decrease resolution due to "memory" effects. Such adsorbed hydrogen is difficult to eliminate and leads to errors in subsequent measurements. Finally, there is no known method which both minimizes adsorption effects and provides accurate and complete information about the isotopic concentration of a sample having a known volume, that is, the relative amounts of H.sub.2, D.sub.2, T.sub.2, HD, HT, and DT found in the sample.